The present invention relates to switching circuits and, more particularly, to switching circuits which sense the status of a load coupled thereto and which provide a flag indicating an open load status.
The prior art is replete with switching circuits in which a load is connected in series with the main electrodes of a transistor between positive and ground power supply rails. By controlling the conduction state of the transistor, current is supplied to the load. One example of such a switching circuit is referred to as a high side driver circuit and is commonly used to control relays in the automotive environment. A typical application of a high side driver circuit is to control the operation fuel injectors of an automotive engine where the load is the low impedance relay coil of the injector. Therefore, by controlling the current through the load, or relay coil, the fuel injector is opened and closed synchronously with the operation of the engine of the automobile as is understood. This is just one example of a use of such a switching circuit; there being myriad of applications for current switching.
In many applications, such as aforedescribed, a power MOSFET switching device has its main electrodes (drain and source) connected in the series conduction path which includes the load, a power supply source and a load current sense resistor. In response to an input signal applied to the circuit, the MOSFET is switched or turned on to complete the series conduction path to permit current flow through the load, the MOSFET and the sense resistor. A 10-30 ampere load current may flow through the series connected elements during normal operating conditions. The sense resistor is used to develop a voltage thereacross the magnitude of which is utilized to limit the maximum value of the load current as is known. In order to maintain maximum load current and to reduce power dissipation, the resistance of the sense resistor is kept very low, i.e., typically less than a half ohm. Thus, in normal operation only a volt or less is developed across the sense resistor at the desired maximum load current.
The foregoing presents a problem if it is desired to be able to detect an open load condition wherein the load current under otherwise normal operating conditions becomes less than several miliamperes. Under an open load condition the voltage developed across the sense resistor may be less than a half milivolt. This small value of voltage may be difficult to accurately detect and to provide the indication of the open load condition.
Thus a need exists to overcome the aforedescribed problems of the prior art.